Enhanced cycling and thermal stability of nickel-rich cathodes for lithium-ion batteries via molecular-weight-tuned polyaniline–polyethylene glycol interphase

Operating Ni-rich layered cathodes like LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811) at high voltages accelerates interfacial degradation, creating significant thermal runaway risks. Here we propose a strategy using a polymer interphase to enhance the intrinsic safety of the cathode material. We synthesized a polyaniline-polyethylene glycol (PANI−PEG) composite coating with optimized molecular weight using a mild solution process that requires no annealing. The pristine and coated cathodes were comprehensively evaluated via electrochemical testing, surface and structural analysis, and simultaneous thermal measurement using thermogravimetric−differential scanning calorimetry (TG−DSC). The results show a clear dependence on PEG molecular weight. The optimized PANI−PEG300 promotes the formation of a mechanically compliant, LiF/Li 2 CO 3 enriched cathode electrolyte interphase, which helps buffer micro-strain accumulation and stabilize the surface chemistry during 2.8−4.4   V cycling. Surface analysis indicates fewer reactive oxygen species after cycling, with the surface active-oxygen fraction decreasing from 27.8% to 13.2%. Importantly, TG−DSC on harvested charged cathodes reveals a lower overall exothermic heat release for PANI−PEG300@NCM811, with ΔH decreasing from 98.313 to 84.557   J/g, together with delayed thermal decomposition, indicating a moderated thermal runaway source term. These changes point to reduced cathode thermal reactivity after high-voltage cycling. This work establishes a quantitative link between polymer molecular design and thermal hazard reduction, offering metrics derived from calorimetry for hazard screening and the inherently safer design of high-energy cathodes.